Separators represent structures in a battery, such as interposed layers, that prevent physical contact of cathodes and anodes while allowing ions to transport therebetween. Separators are formed of materials having pores that provide channels for ion transport, which may include absorbing an electrolyte that contains the ions. Materials for separators are often selected according to softening temperatures, above which, pores rapidly collapse and choke off ion transport. Such softening temperatures may allow separators to arrest operation of the battery when heat generated therein exceeds a safety threshold (e.g., a critical operating temperature).
A shutdown temperature range is typically associated with separators. The shutdown temperature range can be bound by a lower temperature limit and an upper temperature limit. Within the lower temperature limit and the upper temperature limit, the separator offers a high resistance to ion transport. The lower temperature limit corresponds to an onset of pore collapse, which rapidly chokes off ion transport through the separator. Chemical reactions in the battery therefore are arrested and the battery “shuts down”. The upper temperature limit corresponds to a breakdown of the separator, which may include melting and chemical decomposition. At the upper temperature limit, ion-transport begins to increase, which may stem from direct contact between a cathode and an anode.
The battery industry seeks to improve the temperature shutdown range associated with separators. Such improvement may involve decreasing the lower temperature limit, increasing the upper temperature limit, or both.